CZAR 52 Accident aerodynamics
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Hi John,
with regards to what you said about speed and stall, i was wondering, from what ive read at for example 60 degree AOB the effective weight of the aircraft is doubled and the stall speed is increased by 1.41. For eg if an aircraft that has straight and level stall speed of 50 KIAS, during a 60 degree banked turn, due to increase weight/load factor the aircraft now has a stall speed of 70 KIAS.
Is this the correct principle? my way of thinking was that the aircraft at a bank of 45 degrees was travelling at a speed above its "stall speed, however due to slowing down because of the spoiler being in the speed brake 2 position, the aircrafts speed was decreasing and was therefore below its minimum speed for a banked turn at 60 degree.
Or would it have to do with the lower wing reaching its critical angle therefore stalling first as it has a higher AOA than upper wing. I would have thought there was a relationship between speed and stall as in the forumula for lift contains dynamic pressure, therefore if this is not sufficient, not enough lift is produced??
sorry if that is a silly question, im just trying to understand what you are saying and what my txt book seems to say.
Daniel
with regards to what you said about speed and stall, i was wondering, from what ive read at for example 60 degree AOB the effective weight of the aircraft is doubled and the stall speed is increased by 1.41. For eg if an aircraft that has straight and level stall speed of 50 KIAS, during a 60 degree banked turn, due to increase weight/load factor the aircraft now has a stall speed of 70 KIAS.
Is this the correct principle? my way of thinking was that the aircraft at a bank of 45 degrees was travelling at a speed above its "stall speed, however due to slowing down because of the spoiler being in the speed brake 2 position, the aircrafts speed was decreasing and was therefore below its minimum speed for a banked turn at 60 degree.
Or would it have to do with the lower wing reaching its critical angle therefore stalling first as it has a higher AOA than upper wing. I would have thought there was a relationship between speed and stall as in the forumula for lift contains dynamic pressure, therefore if this is not sufficient, not enough lift is produced??
sorry if that is a silly question, im just trying to understand what you are saying and what my txt book seems to say.
Daniel
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perrdan86
What your instructor also failed to take into account was that the a/c had executed a similar manouver up wind, bleeding off any energy that could have extricated him from the final hole. Full flaps (gear down?), and a second energy consuming exercise in high bank angle turns, was his downfall. That and his love affair with looking like the hot pilot he was not. If you notice his record prior, he satisfied himself with a single risky manouver at each meeting. Here at Fairchild, he introduced the "Second Phase", upping the risk. "Breaking new ground" as it were. Following a steep turn with a second (or 'continuation' of the initial one), He was behind in everything, but mostly in energy.
Bonehead.
What your instructor also failed to take into account was that the a/c had executed a similar manouver up wind, bleeding off any energy that could have extricated him from the final hole. Full flaps (gear down?), and a second energy consuming exercise in high bank angle turns, was his downfall. That and his love affair with looking like the hot pilot he was not. If you notice his record prior, he satisfied himself with a single risky manouver at each meeting. Here at Fairchild, he introduced the "Second Phase", upping the risk. "Breaking new ground" as it were. Following a steep turn with a second (or 'continuation' of the initial one), He was behind in everything, but mostly in energy.
Bonehead.
Last edited by bearfoil; 7th Oct 2010 at 00:31.
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You'd best do a search on the flight and find the public versions of the mishap report. It will give you the most accurate description of the event that is available. Those mishap reports are usually quite detailed.
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Daniel,
it was helpful to read the commentary your instructor gave on last year's group, because it indicates to those of us who have university teaching experience the level of student understanding to which the course, and the task, are pitched.
Your comment
touches on a topic which many people (including apparently some professionals) find hard to grasp.
An aircraft can stall at any speed.
The "stall speed" that is defined for many purposes is the speed at which, in level 1g flight, the wing will lose lift because the airflow separates. It is sometimes designated "V_s1g" to emphasise the "1g" condition. Stalling at higher speeds will involve a higher load factor (that is, greater force in the direction of the airplane "z" axis); stalling at lower speeds a lower load factor.
People also say that a wing stalls at a fixed angle of attack. Well, that is also not quite true as stated. The angle of attack at which a wing stalls is dependent on the Mach number at which the aircraft is flying. For example, the stall AoA of many commercial jets in the Mach 0.8-0.9 range is very roughly speaking 7°-10°, whereas at much lower speeds (say, M < 0.2) it is very roughly speaking twice as great.
Also, the point of stall is not necessarily a well-defined point, especially at high speed. It is more like a range of increasingly unpleasant phenomena, and a point is chosen by engineers or test pilots at which the phenomena are "sufficiently unpleasant", and there is some judgement involved in that. At low speeds, at a given load, as with many physical relationships we see in engineering and science (when things go up and then down again), there is an angle of attack at which the lift generated by the wing is maximised. Many people call this the "point of stall", but it is important to understand that there is often plenty of lift generated at higher AoA than this, just not as much as at the maximum!
Some airplanes I have flown, such as the Cessna C-152, have quite a well-defined stall "break", that is, at Vs1g going slower the aerodynamic characteristics change quite abruptly, quite noticeably to the pilot and any passengers (the low-speed lift versus AoA curve "drops off" very sharply after max lift). Then there are other aircraft, such as the Morane-Saulnier Rallye MS880B, which I have just flown and which I anticipate flying quite a lot in the future, in which you notice absolutely nothing abrupt; the airplane just starts to descend (the lift versus AoA curve reduces benignly after max lift).
So when you think about "the wing stalled" as a possible phenomenon to consider in why the B-52 crashed, you would have to know whether loss of lift with the B-52 is a gradual phenomenon, as it is with the MS880B, or a sudden phenomenon, as it is with the C-152. And here's betting you don't know!
Moving on to general considerations. Some contributors here have indicated that they think it is completely clear from the video what is going on aerodynamically. I emphasise again what I said earlier: I don't think it is obvious. Consider: if it were obvious, you would only be getting one uniform opinion from contributors here, whereas you are getting many! Also, if it were obvious then your instructor would likely not be giving you, in advance, his comments on last years answers (for example, a set of comments on answers to the question "What number is 1+1?" is going to be helpful in so far as it consists of the one word "2"!).
Bearfoil touched on a topic which others have not yet mentioned. An airplane without kinetic energy along its x-axis is like anything else: it falls. And if you start out with low energy, and then you generate lift by loading the wing or any other part of the body, then energy is going to dissipate rapidly; it is going to be passed from the airplane into turbulent air movement. And if you run out of energy, aerodynamics is moot, because the "dynamics" part ain't there, which then means, at low level, that the "aero" won't be for long either.
I think this is a good exercise! I hope you are learning a lot!
PBL
it was helpful to read the commentary your instructor gave on last year's group, because it indicates to those of us who have university teaching experience the level of student understanding to which the course, and the task, are pitched.
Your comment
I would have thought there was a relationship between speed and stall
An aircraft can stall at any speed.
The "stall speed" that is defined for many purposes is the speed at which, in level 1g flight, the wing will lose lift because the airflow separates. It is sometimes designated "V_s1g" to emphasise the "1g" condition. Stalling at higher speeds will involve a higher load factor (that is, greater force in the direction of the airplane "z" axis); stalling at lower speeds a lower load factor.
People also say that a wing stalls at a fixed angle of attack. Well, that is also not quite true as stated. The angle of attack at which a wing stalls is dependent on the Mach number at which the aircraft is flying. For example, the stall AoA of many commercial jets in the Mach 0.8-0.9 range is very roughly speaking 7°-10°, whereas at much lower speeds (say, M < 0.2) it is very roughly speaking twice as great.
Also, the point of stall is not necessarily a well-defined point, especially at high speed. It is more like a range of increasingly unpleasant phenomena, and a point is chosen by engineers or test pilots at which the phenomena are "sufficiently unpleasant", and there is some judgement involved in that. At low speeds, at a given load, as with many physical relationships we see in engineering and science (when things go up and then down again), there is an angle of attack at which the lift generated by the wing is maximised. Many people call this the "point of stall", but it is important to understand that there is often plenty of lift generated at higher AoA than this, just not as much as at the maximum!
Some airplanes I have flown, such as the Cessna C-152, have quite a well-defined stall "break", that is, at Vs1g going slower the aerodynamic characteristics change quite abruptly, quite noticeably to the pilot and any passengers (the low-speed lift versus AoA curve "drops off" very sharply after max lift). Then there are other aircraft, such as the Morane-Saulnier Rallye MS880B, which I have just flown and which I anticipate flying quite a lot in the future, in which you notice absolutely nothing abrupt; the airplane just starts to descend (the lift versus AoA curve reduces benignly after max lift).
So when you think about "the wing stalled" as a possible phenomenon to consider in why the B-52 crashed, you would have to know whether loss of lift with the B-52 is a gradual phenomenon, as it is with the MS880B, or a sudden phenomenon, as it is with the C-152. And here's betting you don't know!
Moving on to general considerations. Some contributors here have indicated that they think it is completely clear from the video what is going on aerodynamically. I emphasise again what I said earlier: I don't think it is obvious. Consider: if it were obvious, you would only be getting one uniform opinion from contributors here, whereas you are getting many! Also, if it were obvious then your instructor would likely not be giving you, in advance, his comments on last years answers (for example, a set of comments on answers to the question "What number is 1+1?" is going to be helpful in so far as it consists of the one word "2"!).
Bearfoil touched on a topic which others have not yet mentioned. An airplane without kinetic energy along its x-axis is like anything else: it falls. And if you start out with low energy, and then you generate lift by loading the wing or any other part of the body, then energy is going to dissipate rapidly; it is going to be passed from the airplane into turbulent air movement. And if you run out of energy, aerodynamics is moot, because the "dynamics" part ain't there, which then means, at low level, that the "aero" won't be for long either.
I think this is a good exercise! I hope you are learning a lot!
PBL
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Stall Stick Position 
You have to pull to get a stall, generally speaking at a constant speed the lift that you can create with the wing increases linearly with angle of attack which you increase by pulling back on the stick. It reaches a maximum at the stall where there is a marked increase in airflow separation over the wing. Roll alone cannot cause a stall. Otherwise how would aerobatic aircraft do 360 degree rolls with out falling out of the sky.
You have to pull to get a stall, generally speaking at a constant speed the lift that you can create with the wing increases linearly with angle of attack which you increase by pulling back on the stick. It reaches a maximum at the stall where there is a marked increase in airflow separation over the wing. Roll alone cannot cause a stall. Otherwise how would aerobatic aircraft do 360 degree rolls with out falling out of the sky.
Moderator
We've combined the OP's three or so threads on this subject (very naughty to have multiple threads going simultaneously).
Hopefully the edits have not made the result difficult to follow ?
Hopefully the edits have not made the result difficult to follow ?
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Also a bit naughty of contributors to quote the answer extensively from investigative documents when the OP had identified it as a study project. Explanations, clarifications and hints from third parties are, for most teachers, welcome, and part of the learning process. Answers are not!
PBL
PBL
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86
you only stall at 1.414 times the wings level stall speed at 60 AOB IF you pull the stick back and try to hold height (ie actually pull the required g such that the vertical component of the lift vector equals the weight).
Sorry if you don't understand this.
Only if you DO hold height with 60 deg bank will you increase the lift vector to a length of double the weight (needed to keep the VERTICAL component of the lift vector equal to the weight) and so put up your stall speed to 1.414 times the wing level case.
Rolling to a high banck angle without pulling back and increasing the AOA will NOT affect the stall margin. What will happen to your flight path is another matter though.
Perrdan86, if you have the time or inclination ,why not find a `flying school`,and go and speak to the CFI,or even a simulator,not a microsoft one, to do a practical sortie, when you have looked at all the relevant information,and done some analysis.( I`m assuming you have`nt done any practical flying ).It doesn`t have to be in an exotic jet,as the same accident occurs to people who don`t pay attention to what the aircraft is `telling you`,either thru` `fixation outside`,trying to line-up by overbanking,pulling too much in a descending spiral instead of `rolling-out`,then pulling,or any combination.
We normally say ` stall ,flick ,spin,crash, die !`; it happens in Piper Cubs,to B-52s.......
edit later; having watched the `u-tube` videos,at that height there was no going back ,after about 60 deg. of bank,and there was little or no power applied,going by the smoke trail,compared to the turn after initial t/off. There was full opposite spoiler applied, but the nose had gone down,and it appears little `g` applied(wing-flex),and it appears full flap,with lots of drag...
We normally say ` stall ,flick ,spin,crash, die !`; it happens in Piper Cubs,to B-52s.......
edit later; having watched the `u-tube` videos,at that height there was no going back ,after about 60 deg. of bank,and there was little or no power applied,going by the smoke trail,compared to the turn after initial t/off. There was full opposite spoiler applied, but the nose had gone down,and it appears little `g` applied(wing-flex),and it appears full flap,with lots of drag...
Last edited by sycamore; 7th Oct 2010 at 18:00.
